TiO2

Accepted Manuscript Title: Enhanced visible light catalytic activity of MoS2 /TiO2 /Ti photocathode by hybrid-junction Authors: Chaoqun Cheng, Guohua Liu, Kang Du, Gang Li, Wendong Zhang, Simone Sanna, Yunzhong Chen, Nini Pryds, Kaiying Wang PII: DOI: Reference:

S0926-3373(18)30538-1 https://doi.org/10.1016/j.apcatb.2018.06.012 APCATB 16760

To appear in:

Applied Catalysis B: Environmental

Received date: Revised date: Accepted date:

15-4-2018 31-5-2018 3-6-2018

Please cite this article as: Cheng C, Liu G, Du K, Li G, Zhang W, Sanna S, Chen Y, Pryds N, Wang K, Enhanced visible light catalytic activity of MoS2 /TiO2 /Ti photocathode by hybrid-junction, Applied Catalysis B: Environmental (2018), https://doi.org/10.1016/j.apcatb.2018.06.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced

visible

light

catalytic

activity

of

MoS2/TiO2/Ti

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photocathode by hybrid-junction Chaoqun Chenga, Guohua Liub, Kang Dua, Gang Lic, Wendong Zhangc, Simone Sannad, Yunzhong Chend, Nini Prydsd and Kaiying Wanga,*

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Department of MicroSystems, University College of Southeast Norway, Horten 3184, Norway State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources,

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b

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a

Micro and Nano System Research Center, Key Lab of Advanced Transducers and Intelligent

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c

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North China Electric Power University, Beijing 102206, China

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Control System (Ministry of Education) & College of Information Engineering, Taiyuan

d

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University of Technology, Taiyuan, 030024, China. Department of energy conversion and storage, Technical University of Denmark, Risø Campus,

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4000 Roskilde, Denmark

Corresponding Author *E-mail: [email protected]

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Graphical Abstract

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Highlights

MoS2/TiO2/Ti photocathode was facile fabricated with visible light responsivity.

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Enhanced catalytic activity under visible light illumination was achieved.

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The enhancement was due to improved charge separation, transfer and transport.

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A proper charge-carrier dynamics of hybrid-junction was proposed.

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Abstract:

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In photoelectrochemical (PEC) water splitting systems, crucial obstacles limiting their performance are poor charge carrier dynamics and high recombination rate of photoexcited electron hole pairs. Here, we report that this issue can be alleviated by engineering a hybridjunction that is composed of homo- and hetero- junctions. This strategy is performed by facile hand-spraying MoS2 over the surface of a anatase/rutile homo-junction TiO2 film on the Ti

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substrate to further form a hybrid-junction photocathode. By applying this photocathode into PEC reactor, enhanced catalytic activity is achieved under visible light (AM1.5 illumination of 300 W/m2) with hydrogen evolution reaction (HER) potential of -114 mV versus reversible hydrogen

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electrode (RHE) at 10 mA/cm2 and long-term stability of more than 10 times improvement comparing to ordinary electrode without the introduciton of hybrid-junction. The hybrid-junction that effectively regulates charge separation and transfer pathways is proven to be responsible for the enhanced activity. As an novel exploration, this hybrid-junction system comprising of lowcost, efficient charge separation and transfer, and visible light responsivity offers a new path for

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relative materials to boost their PEC performance.

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KEYWORDS:

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hybrid-junction, water splitting, titanium dioxide, Z-scheme, molybdenum disulfide

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1. Introduction

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Photoelectrochemical (PEC) water splitting system harvesting sunlight to drive uphill water

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splitting reactions has been the focus of attention due to its potential of converting sunlight into chemical fuels in a cost-economical and sustainable way

[1-4]

. In the past decades excellent

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advancement has been achieved, specially the metal oxide semiconductor has been invested greatly due to their intrinsic nature of the d orbital states, earth-abundance and superior PEC performance . However, their PEC performance to the practical commercialization is still subjected to the

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[5-8]

limitations of poor charge-carrier dynamics and high recombination rate issues under the solar illumination [9,10]. To solve these issues, some efforts have been made to pursue reliable strategies in particular the search for hybrid systems [3,11-13].

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Hetero- and homo-junctions are two distinctive approaches used in the hybrid systems[14,15]. The designing of hetero-junction is a well-established approach as it holds great potential to overcome the drawbacks of serious charge recombination and limited visible-light utilization

[16]

, typically

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involving building an advanced two-step (Z-scheme) system. The Z-scheme system composed of two semiconductors with desired band gaps and energy positions ensure that the separated electrons and holes transport only in opposite directions toward surface for preferred reactions, minimizing deleterious recombination and undesirable side reactions [17, 18]. Z-scheme construction can not only regulate charge transfer pathways but also increase the sunlight utilization through

. As an example, there have been reports that Z-scheme hetero-junction of MoS2/TiO2 with

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[19,20]

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the series connected two semiconducting materials with narrow bandgap absorbing visible light

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CdS quantum dots can effectively improve the catalytic activities under the visible light irradiation [21]

. Recent studies have also

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as the result of beneficial charge interaction in the interface

demonstrated that the excellent catalytic activity in the heterojunctions of MoS2/TiO2 is related to

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the strong interaction between the MoS2 and TiO2 [22-27].

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On the other hand, the construction of homo-junction (phase-junction) is an alternative

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configuration access to charge separation because the formation of homo-junction between two distinct phases can adjust the transport of the charge carriers along the inter-phases band alignment . For example, it has been reported that surficial phase-junction between the nanoparticles of

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[9]

anatase and rutile TiO2 can enhance the water splitting activities [28]. The basic mechanism is that

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the surficial anatase/rutile phase junctions can guide the electrons or holes transferred between band structures of rutile and anatase TiO2, thereby facilitating separation of electron-hole pairs and catalytic activities [28-30].

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Although configuring hetero-junction or homo-junction is well explored, the potentials of integrating hybrid-junction comprising of hetero-junction and homo-junction into a single photoelectrode is still lack of investigation, especially in terms of the impact on charge carrier [31-33]

was explored to

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dynamics. In the heterogeneous system, the doping of foreign elements

provide multi-channel paths for improving the charge-carrier dynamics. In this study, we propose a facile strategy of building hybrid-junction to improve the catalytic activity of photocathode under visible light. We demonstrate that integration of homo-junction and hetero-junction on the bulk metal substrate as a simple but effective strategy could effectively fast the performance of charge

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separation, transfer and transportation dynamics at the interface and reduce charge carrier

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recombination in the bulk and at the surface.

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We fabricated hybrid-junction of MoS2/TiO2 by hetero-coupling MoS2 and homo-junction

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(mixed-phases) TiO2 on the substrate of Ti foil. The hybrid-junction MoS2/TiO2 exhibits high activities under a visible light irradiation (AM 1.5G illumination of 300 W/m2) with hydrogen

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evolution reaction (HER) potential of -114 mV versus reversible hydrogen electrode (RHE) at 10

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mA/cm2, as well as long-term stability for more than 12 hours at -0.3 V vs RHE while comparing

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to about 1 hour without the introduction of hybrid-junction under same measurement conditions, more than 10 times improvement. The homo-junction TiO2 thin layer on the substrate of Ti was

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synthesized through simple one-step thermal annealing process, different from traditional synthesis using hydrothermal or sol-gel methods [34-36]. The characterization on the HER activities

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suggest the homo-junction TiO2 on the substrate of Ti could facilitate charge separation and transportation under the visible light illumination. After the hetero-coupling with MoS2 constructed as hybrid-junction system, this cathode composite of MoS2/TiO2 on the substrate Ti exhibit higher activity and stability during the HER. The underlying charge transfer analysis

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elucidated that the hybrid-junction of Z-scheme hetero-junction and homo-junction plays important roles in the excellent performance. 2. Experimental section

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2.1. Materials Commercially Molybdenum disulfide (MoS2) microflakes liquid-spraying was purchased from CRC Industries Americas Group. Titanium foils (Ti foils, 30 mm × 14 mm × 0.5 mm, 99.8 % purity) were cut out from Ti sheet, which was purchased from Baoji Titanium Industry Co., LTD. Ethanol, acetone, isopropanol and deionized water were obtained from Sigma-Aldrich without

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further purification.

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2.2. Preparation of homo-junction TiO2 and MoS2/TiO2 composite electrodes

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The fabrication consists of oxidation, spraying and annealing processes. Initially, Ti foils were

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cleaned by immersing the foils into the acetone rinsed by ultrasonic bath and subsequently dried in flowing nitrogen gas. After the initial cleaning step, Ti foils were oxidized at 400, 500 and 600 ℃

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for 2 hours with oxygen gas continually flowing through the quartz tube furnace to generate a thin

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layer of homo-junction TiO2 on the surface of the Ti foil (Figure S1). Following the oxidation step,

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commercial lubricating MoS2 spray (CRC Industries) was applied to the TiO2/Ti, followed by baking at 100 ℃ in the oven with ambient atmosphere for 10 hours to ensure that all the solvent is

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evaporated. In the last annealing process, the pristine MoS2/TiO2/Ti foils were annealed at 500 ℃ for 1, 2, and 3 hours, respectively in Nitrogen atmosphere under ambient pressure and cooled to

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the room temperature. 2.3. Physical Characterization Surface morphology of as-prepared samples was examined by field emission scanning electron microscopy (FESEM) (Hitachi, SU 8230) equipped with energy-dispersive X-ray spectrum (EDS)

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and electron backscatter diffraction (EBSD) microstructural-crystallographic characterization. Atomic force microscopy (AFM, Parker XE 200) was used to visualize surface the topography of samples on the nanometer scale. UV-Vis light absorption spectra of the as-prepared samples were

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obtained using a SHIMADZU, UV-2600 spectrophotometer equipped with an integrated sphere assembly using diffuse reflection method. BaSO4 was used as a reference to measure all samples in the wavelength range of 250 - 800 nm with a slit width of 1 nm. X-ray diffraction (XRD) patterns were collected on a Bruker D8-Advance diffractometer using Cu Kα radiation (λ = 1.5405 Å) in a range of 2-80° at a scan rate of 5°/min. Photoluminescence (PL) spectra were performed in the

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FS5 Spectrofluorometer of Edinburgh Instruments by focusing the laser radiation centered at 400

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nm from an Xeon lamp onto the samples.

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2.4. Photoelectrochemical Measurement

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PEC/Electrochemical measurements with/without simulated solar light illumination were performed and evaluated in a standard PEC three electrodes setup which is connected to

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electrochemical workstation (Zahner Elektrik IM6). The incident light was filtered through AM

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1.5G solar simulator over an adjustable 500 W Xeon lamp (AM 1.5G, 300 W/m2). The as-prepared

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composites, platinum gauze and Ag/AgCl with saturated KCl solution were used as working, counter and reference electrodes. In the text, the measured potentials vs. Ag/AgCl were all changed

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to the RHE scale based on the Nernst equation: ERHE=EAg/EAgCl + pH * 0.059 + 0.1976. All the current density reported in the work were calibrated using IR compensation, where the R

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(resistivity) was estimated by electrochemical impedance spectroscopy (EIS) with a frequency ranging from 10 Hz to 1 M Hz. Linear scanning voltammetry measurements were carried out in 0.5 M H2SO4 at a constant scan rate of 20 mV/s over the potential range of -0.6 - 0 V (vs RHE). A long-term stability test for

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MoS2/TiO2 and MoS2/Ti were conducted by an extended electrolysis (15 hours) under dark or light conditions at a constant reduction potential of -0.3 V vs RHE. EIS test were carried out at opencircuit potential by applying an AC potential with amplitude of 50 mV during the frequency range

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of 4 × 105 to 10-3 Hz in 0.5 M H2SO4 with/without the AM 1.5G illumination (300 W/m2). MottSchottky plots were obtained at frequencies of 5, 10 and 15 kHz and amplitude of 10 mV in 0.5 M H2SO4 solution under the dark or light conditions. In addition, current density-voltage (J-V) characteristic in an anodic direction was acquired at a scan rate of 20 mV/s. The measurements were carried out in a solution of Na2SO4 (pH 6.8) and subsequently in another solution of 1 M

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KOH (pH 13).

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3.1. Homo-junction TiO2 on the substrate of Ti

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3. Results and discussion

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Homo-junction TiO2 was synthesized on the surface of Ti foil using simple one-step annealing process, as presented in the experimental section. The presence of homo-junction (mixed-phases)

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TiO2 on the surface of Ti foil after the annealing process was determined by X-ray diffraction

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(XRD) and electron backscatter diffraction (EBSD). Figure 1a shows the XRD patterns of TiO2 at

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temperature ranging from 400 ℃ to 600 ℃, in which both anatase or rutile TiO2 could be observed (Figure S2). Figure 1b presents the EBSD mapping, clearly showing the mixed-phases orientation

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mapping on the surface with assorted colors. Figure 1c shows the UV-Vis absorption spectra, indicating a strong resonant visible light

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absorption peak around 480 nm at 500 ℃ [37-40]. Figure 1d further shows the UV-vis absorption spectra, in which the absorption edge wavelength of material moves to the infrared light area in the direction of longer wavelength as the processing temperature increases gradually, confirming the dependence of resonance absorption wavelength on the microscopic changes of surficial

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mixed-phases TiO2 induced by the elevated thermal treatment in the temperature range (Figure S4) [37, 41]

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Figure 2 show J-V plots of proton reduction with/without AM 1.5G illumination and

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corresponding Tafel slope values. Figures 2a-c show J-V curves measured in dark and light respectively. As seen from these figures, TiO2/Ti (400 ℃) and TiO2/Ti (500 ℃) exhibit better reduction activities under the light condition, while TiO2/Ti (600 ℃) itself shows a poor reduction current under light, even worse than that in dark. This difference is consistent with the differential absorption spectra in the Figure 1c. For the TiO2/Ti (500 ℃), an overpotential of 216 mV at 10

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mV/cm2 was achieved under the application of light, almost 50% improvement comparing to the

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403 mV under the dark condition, suggesting that TiO2/Ti (500) has better PEC activities in the

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visible light range. Figure 2d presents the Tafel slope for the TiO2/Ti under the dark and light, in

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which the TiO2/Ti (500 ℃) has a lower Tafel slope value of 159 mV/decade, and thus was better activity than the others. Since the Tafel slope value is close to the Volmer reaction regime (120

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amount of surface active sites.

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mV/decade) [42,43], the rate-limiting step here is the electrochemical adsorption step due to a limited

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3.2. Hybrid-junction MoS2/TiO2/Ti electrode The investigation suggests that the homo-junction TiO2 on the substrate of Ti has favourable

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charge transfer features under the visible light illumination but is restricted by the deficient surface sites which could increase the rate of interfacial charge transport. To eliminate the deficit, the

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homo-junction TiO2 was covered by hand-spraying thin layer of commercial MoS2, forming a functional complementary hetero-junction system. In addition to the merits of containing no precious metals and earth abundance, MoS2 with large amount of edges could also offer enough active sites for HER activities [7,44]. The commercial MoS2 nanoflakes liquid-spraying received our

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attention due to the universal of this method enabling easy-fabricating hetero-junction system. Figure 3(a) shows the facile and cost-efficient processes for fabricating the novel hybrid-junction MoS2/TiO2. All the prepared samples at different processes are noted respectively as Ti (A), homo-

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junction TiO2/Ti (B), pristine MoS2/TiO2/Ti (C) and annealed MoS2/TiO2/Ti (1h/2h/3h) (D/E/F). Figure 3(b) shows SEM top-view images of the TiO2 (sample B), pristine MoS2/TiO2/Ti (noted as MoS2/TiO2, sample C) and annealed MoS2/TiO2/Ti (MoS2/TiO2, sample D, E and F). The morphology of the materials TiO2 on the surface is shown in Figure 3 (b)-B. The as-received (without thermal treatment) pristine MoS2 shows rough and irregular surface morphology. Upon

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annealing at 500 ℃ in the nitrogen atmosphere, much more dense structure with small pores on the

[43]

. Favouring the specific surface area, these top-porous networks

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surficial MoS2 microflakes

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surface is observed in the sample E, mainly due to the thermal treatment which crystalizes the

[45]

. Furthermore, the thin layer of continuous

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offer more edge sites allowing for HER reactions

network structure on the surface provides an accessible tunnel for light penetrating to the inner

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part of TiO2. After longer annealing time, the undersigned damage to the surface structure could

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be observed as the potholes in the surface of sample F. Figure S6 show the energy dispersive X-

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ray spectroscopy (EDS) mapping, confirming that the Ti, O, Mo, and S species existed in the multilayered film composites. To verify the thickness of MoS2 and the MoS2 wrapped on the surface of

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particles, a cross-sectional FESEM analysis was conducted (Figure S7a, b). The FESEM images show distinct wrapping of MoS2, with measured thickness of 1.30 um (MoS2 pristine) and 400 nm

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(MoS2 2h). Element-specific analysis was performed by the dedicated Line-scan EDS along the direction of bottom to surface (Figure 1c), indicating existence of the material of MoS2. The morphology of MoS2 wrapped on the surface of particles was further confirmed by AFM surface topology (Figure S7d).

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Figure 3(c) shows the XRD patterns of five as-prepared hybrid-junction samples. Before spraying MoS2, the peaks of (112) and (204) facets of anatase TiO2 and (111) facet of rutile TiO2 could be observed in Figure 3(c)-B (Anatase PDF 00-021-1272; Rutile PDF 00-021-1276). After

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annealing, the diffraction characteristic peaks of TiO2 are still observed in the patterns of MoS2/TiO2 composites with grown crystallite size and strength after the thermal treatment. The peaks centered at 26° and 28.7° can be assigned to (101) facet of anatase TiO2 and (110) facet of rutile TiO2 (Anatase PDF 00-021-1272; Rutile PDF 00-021-1276). For MoS2, the detected signals located at 29.5°, 44.6° and 60.5° are assigned to (004), (006) and (110) (MoS2 PDF 01-087-2416) [46]

. The

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respectively with increased crystallinity along with increasing annealing temperature

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crystal phase information on the TiO2 and MoS2 suggests the hetero-junction composite consisting

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of MoS2 and mixed-phases TiO2 based homo-junction.

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The HER activities of MoS2/TiO2 were measured and compared with that of the MoS2/Ti. Figure 4a presents photo-current of the obtained dark-current and photo-current (Figure S8(a-c)), in which

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the MoS2/TiO2 (500 ℃ 2h) exhibited higher photo-current during the HER under the light

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irradiation than the rest, and excellent HER activities with overpotential of -114 mV at 10 mA/cm2.

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The improved overpotential is much lower than that of latest reports, such as 0D(MoS 2)/2D(gC3N4) heterojunctions (about -450 mV at 2 mA/cm2) [18], a-MoSx@MPA colloidal nanodots (about [47]

and high-percentage 1T-phase MoS2 nanodots (about -180 mV at 10

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-270 mV at 5 mA/cm2)

mA/cm2) [48]. It is even comparable to that of MoS2 with multifunctional active sites (around -100

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mV at 10 mA/cm2 in 1.0 M aqueous KOH) [49]. To examine the role of homo-junction TiO2 in the hetero-junction, the HER activities of MoS2/Ti without introducing homo-junction TiO2 was also measured. Figure S8d shows the current density under the dark and light condition during the HER for the MoS2/Ti. The current density under the light is almost overlapped with the current density

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under the dark, indicating a poor visible light utilization. Thus, the homo-junction TiO2 plays a crucial role on promoting the HER activities of MoS2/TiO2 after thermal treatment. Figure 4b presents the Tafel slope for the MoS2/TiO2 (500 ℃ 2h), with a value of 99 mV/decade under the

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light, much lower than the Tafel slope value 159 mV/decade of TiO2/Ti shown in the Figure 2d. The Tafel slope value is much closer to the Heyrovsky reaction regime (40 mV/decade), therefore the rate-limiting step is changing from the electrochemical adsorption step to the desorption step due to the covered MoS2 [43]. In addition, in contrast to the HER activities of singular TiO2 (TiO2/Ti, Figure 2c) and MoS2 (MoS2/Ti, Figure S8d), the hetero-junction of MoS2/TiO2 exhibits lower

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overpotential and higher current under the light, suggesting that the hetero-junction interface

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between the homo-junction TiO2 and MoS2 also contributed to the charge separation and transfer,

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and then hastened the reaction kinetics.

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A long-time stability test for MoS2/TiO2 (500 ℃ 2h) was also carried out. Figure 4c shows the current density measured during an extended electrolysis (15 hours) under the light at constant

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potential of -0.3 V (vs RHE), in which only 5% current density decay was observed after 12-hours

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continuous test under the light for the MoS2/TiO2 (500 ℃ 2h). For comparison, Figure 4d and

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Figure 4e present the stability test for both MoS2/TiO2 without illumination and MoS2/Ti with illumination. Almost 30% current decay was obtained for the MoS2/Ti under the light,

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demonstrating a faster degradation rate than that of MoS2/TiO2 with/without illumination. Figure 4f further shows the HER polarization curves before and after 10 hours stability test for

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the MoS2/TiO2 with/without the light, in which the current curve under the dark after 10 hours stability measurement displays similar and repeatable features as the initial test before stability test. While a larger deviation in the current was observed under the light condition, indicating that the stability under the light is slightly weaker than that under the dark. We attribute the reason for the

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stability difference between the dark and the light to the photoinduced corrosion process on the edge sites or defect sites of the MoS2 under the light exposure [50], because the faster and stronger reactions happened during the HER under the light. EIS was used to examine the impedance

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difference. Figure S9 shows the EIS plots, in which a smaller semicircle of the arc line was exhibited for the MoS2/TiO2 (500 ℃ 2h) under the light comparing to that under the dark, indicating better interfacial charge-transfer kinetics [43,44]. Meanwhile, a concave arc-line representing intense diffusion of ions of the interface between electrolyte and electrode was observed for the MoS 2/Ti under the light [51,52], suggesting poor charge-transfer property which is in agreement with a weak

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PEC activities. It should be noted that the high stability of the hybrid-junction of MoS2/TiO2 is

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comparable to other reported junctions such as MoS2/TiO2, nanojunction of (001) Facets

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MoS2/Anatase TiO2 (cyclic H2 production for 9 hours) [53], heterojunction of MoS2/anatase TiO2

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(recycle degradation of MB for 5 hours) [4], and MoS2/TiO2 Edge-On Heterostructure (12 hours recycling photocatalytic H2 evolution with intermittent evacuation) [26].

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3.3. Discussion

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Figure S10 a,b shows the UV-Vis absorption spectra of TiO2 and MoS2 respectively, in which

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the absorption edges of TiO2 and MoS2 were found to be 760 and 990 nm respectively, corresponding to the direct band gap energy (Eg) of 1.63 eV and 1.25 eV derived from Beer– [37]

as shown in the Figure S10c. The narrower bandgap energy of homo-junction

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Lambert law

TiO2 compare with the pure anatase phase (3.20 eV) and rutile phase of TiO2 (3.03 eV) [54] could

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be attributed to the monolithic homo-junction TiO2. To further understand the band positions, Figure S11 presents the Mott-Schottky plots of the TiO2 and the MoS2/TiO2 (500 ℃ 2 h) (the inset in Figure S11), in which the positive slopes of Mott-Schottky indicate n-type TiO2 and MoS2 55. The labeled flat-band potentials are pointing at the difference between Fermi levels and water-

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oxidation potentials. We assumed that the Fermi levels were close to the conduction band, ignoring the slight difference between the Fermi levels and conduction band which has no effect on the analytical conclusion. Based on this, we propose an energy diagram of charge-carrier generation,

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separation and migration processes for the hybrid-junction MoS2/TiO2 on the Ti substrate as illustrated in Figure 5. The detailed Z-scheme mechanism are illustrated in the Figure S12 and explained as follows.

In general, the MoS2/TiO2 composite shall be working as typical staggered gap (type II) heterojunction under usual double paths of charge transferring

[56-59]

. Guided by the matched

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bending band between the MoS2 and TiO2, an inner potential difference induced electric field

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would be created. The electrons from the conduction band (CB) of MoS2 could transfer to the CB

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of TiO2; in the meantime, the holes would tend to transfer from the valance band (VB) of TiO2 to

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the VB of MoS2. Given the fact that the CB potential of homo-junction TiO2 is more positive than the standard proton reduction potential of H2/H2O (0.00 V Vs NHE), thereby protons cannot be

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thermodynamically reduced to hydrogen. Similarly, since the VB potential of MoS 2 is more

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negative than that of O2/H2O (1.23 V Vs NHE), thus the oxygen evolution reaction is prohibited.

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However, enhanced HER activities and stability were achieved in the work. Therefore, it is proposed that a direct Z-scheme catalytic mechanism is established. Under the mechanism, the

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built-in static electric field would guide the electrons from the CB of homo-junction TiO2 transferring to the interface to combine with the active holes from the VB of MoS2, thus preventing

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the electrons in the CB of homo-junction TiO2 from the strong reducibility and the holes in the VB of MoS2 from the oxidizability. Thus, an enhanced HER activities could be achieved on the hybridjunction of MoS2/TiO2. [60]

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For the hetero-junction of MoS2/TiO2, to further ascertain that the underneath homo-junction TiO2 on the substrate of Ti still has resonant visible light responsivity during the HER, we applied the linear sweeps (J-V) in anodic direction as shown in Figure S13 under the dark. The oxygen

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evolution reactions (OER) was observed as gas bubbles in the electrolytes. Note that, once the gas bubbles were generated out of the surface, the surficial MoS2 would delaminate from the surface due to the fact that the OER just taking place on the active sites of inner TiO2. Once irradiated by light, the observed delamination process became much more severe so that deteriorating of MoS2 flakes was observed. The reason could be due to the TiO2 harvesting light energy resulted in the

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accelerated OER. These findings confirm that the ions and visible light can interacting with the

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underneath TiO2, possibly owing to surficial porous MoS2. Therefore, the underneath homo-

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junction TiO2 on the substrate of Ti could absorb the visible light and convert the light energy

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during the catalytic reactions.

The interface between the TiO2 and Ti in the hetero-junction of MoS2/TiO2/Ti also plays an

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important effect on the overall charge carriers transfer and separation while restraining the

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recombination which resulted in the photoluminescence (PL) signal. Thus, PL emission spectra

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were recorded (Figure S14) to probe the behavior of the interface. Comparing to the PL intensity of Ti substrate, much weaker intensity of MoS2/Ti, TiO2/Ti and MoS2/TiO2/Ti were obtained,

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suggesting the surficial decoration with MoS2, TiO2, or MoS2/TiO2 could depress the recombination effectively [61]. This confirms the interface is in favor of the charge separation and

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transfer between the TiO2 and Ti during the HER under the light. In brief, the homo-junction on the substrate of Ti and Z-scheme hetero-junction play multiple

roles during the whole HER. First, the homo-junction TiO2 with narrow band gap can not only utilize visible light energy to excite the electrons from the VB to CB, but also acquire the hot

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electrons transferred at the interface of quasi-core-shell TiO2/Ti [37-40]. The homo-junction forms an inter-phase junction to afford the band bending, and then favoring the excited electrons toward the hetero-junction interface. Second, the Z-scheme hetero-junction transports the electrons to the

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VB of MoS2, where the electrons are then excited to the CB by the sunlight energy, further participating into the HER on the active sites of MoS2. Furthermore, the Z-scheme connection provides excellent protection on the MoS2 through directing the photo-excited holes in the VB of MoS2 toward the hetero-junction interface space meeting with the electrons from the CB of TiO2, preventing the MoS2 from the photo-excited holes induced corrosion and therefore imparting a

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higher stability to the system [62]. In turn, the upper MoS2 layer also acts as protective material for

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the underneath TiO2. The low cost of Ti and MoS2 with simply fabrication processes makes them

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favorable to demonstrate our hybrid-junction scheme. The optimization for the processes and

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manipulation should result in better HER performance and enhanced the visible light utilization,

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Conclusions

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like modifying the thickness of MoS2 [63-65].

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Novel hybrid-junction MoS2/TiO2/Ti photocathode was fabricated by a facile process that combines thermal oxidaiton and hand-spraying. This hybrid-junction photocathode under visible

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light exhibited improved HER activities, in which the overpotential required to obtain 10 mA cmwas reduced to -114 mV, as well as long-term stability was achieved for over more than 12 hours

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at -0.3 V Vs RHE while in comparison the cathode without the introduction of hybrid-junction only persisted about 1 hour under the same conditions. The associated homo-junction and Zscheme hetero-junction were attributed to the enhanced charge carrier dynamics and reduced recombination, therefore improved HER activities under the light. Synthesized by simple one-step

16

thermal oxidation process, the homo-junction TiO2 on the substrate of Ti with plasmonic resonances and charge separation ability is supposed to have more wide range of application capability in sunlight energy-conversion systems. The concept of hybrid-junction provides a cost-

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economical strategy to regulate the charge carrier separation, transfer and transportation dynamics of photoelectrodes and catalysts that do hold poor characteristic under given working environment for practical solar water splitting.

ACKNOWLEDGEMENT

Programme

(231416/F20),

China

Scholarship

N

FRINATEK

U

The authors would like to acknowledge the support from Norwegian Research Council Council

(CSC,

Grant

A

No.201506930002), the Norwegian Micro- and Nano-Fabrication Facility (NorFab, project

M

number 245963/F50), and a STSM Grant from To-BE COST Action (MP 1308) supported by

D

COST (European Cooperation in Science and Technology). We also want to acknowledge the

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valuable help of Muhammad Tayyib who helped us in the characterization of SEM, EDX and

A

CC

EP

EBSD.

17

REFERENCES [1] O. Khaselev, A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting, Science, 280 (1998) 425-427.

SC RI PT

[2] Y. Tachibana, L. Vayssieres, J.R. Durrant, Artificial photosynthesis for solar watersplitting, Nat. Photonics, 6 (2012) 511-518.

[3] D.M. Fabian, S. Hu, N. Singh, F.A. Houle, T. Hisatomi, K. Domen, F.E. Osterloh, S. Ardo, Particle suspension reactors and materials for solar-driven water splitting, Energy Environ. Sci., 8 (2015) 2825-2850.

U

[4] A. Landman, H. Dotan, G.E. Shter, M. Wullenkord, A. Houaijia, A. Maljusch, G.S. Grader,

N

A. Rothschild, Photoelectrochemical water splitting in separate oxygen and hydrogen cells,

A

Nat. Mater., 16 (2017) 646-651.

M

[5] R.M. Navarro, M.C. Alvarez-Galván, J.A. Villoria de la Mano, S.M. Al-Zahrani, J.L.G. Fierro, A framework for visible-light water splitting, Energy Environ. Sci., 3 (2010) 1865.

D

[6] J.H. Montoya, L.C. Seitz, P. Chakthranont, A. Vojvodic, T.F. Jaramillo, J.K. Norskov,

TE

Materials for solar fuels and chemicals, Nat. Mater., 16 (2016) 70-81.

EP

[7] I. Roger, M.A. Shipman, M.D. Symes, Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting, Nat. Rev. Chem., 1 (2017) 0003.

CC

[8] L. Zhou, H. Zhang, H. Sun, S. Liu, M.O. Tade, S. Wang, W. Jin, Recent advances in non-

A

metal modification of graphitic carbon nitride for photocatalysis: a historic review, Catal. Sci. Technol., 6 (2016) 7002-7023.

[9] F.F. Abdi, L. Han, A.H. Smets, M. Zeman, B. Dam, R. van de Krol, Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode, Nat. Commun., 4 (2013) 2195. Khaselev

18

[10] K.T. Fountaine, H.J. Lewerenz, H.A. Atwater, Efficiency limits for photoelectrochemical water-splitting, Nat. Commun., 7 (2016) 13706. [11] H.M. Chen, C.K. Chen, R.S. Liu, L. Zhang, J. Zhang, D.P. Wilkinson, Nano-architecture

SC RI PT

and material designs for water splitting photoelectrodes, Chem. Soc. Rev., 41 (2012) 5654-5671.

[12] S.Y. Reece, J.A. Hamel, K. Sung, T.D. Jarvi, A.J. Esswein, J.J. Pijpers, D.G. Nocera, Wireless solar water splitting using silicon-based semiconductors and earth-abundant catalysts, Science, 334 (2011) 645-648.

U

[13] J. Luo, J.H. Im, M.T. Mayer, M. Schreier, M.K. Nazeeruddin, N.G. Park, S.D. Tilley, H.J.

N

Fan, M. Gratzel, Water photolysis at 12.3% efficiency via perovskite photovoltaics and

A

Earth-abundant catalysts, Science, 345 (2014) 1593-1596.

M

[14] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, Engineering heterogeneous semiconductors for solar water splitting, J. Mater. Chem. A, 3 (2015) 2485-2534.

D

[15] L. Yuan, C. Han, M.-Q. Yang, Y.-J. Xu, Photocatalytic water splitting for solar hydrogen

TE

generation: fundamentals and recent advancements, Int. Rev. Phys. Chem., 35 (2016) 1-

EP

36.

[16] H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor

CC

heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev., 43 (2014) 5234-5244.

A

[17] G. Liu, K. Du, S. Haussener, K. Wang, Charge transport in two-photon semiconducting structures for solar fuels, ChemSusChem, 9 (2016) 2878-2904.

19

[18] Y. Liu, H. Zhang, J. Ke, J. Zhang, W. Tian, X. Xu, X. Duan, H. Sun, M. O Tade, S. Wang, 0D (MoS2)/2D (g-C3N4) heterojunctions in Z-scheme for enhanced photocatalytic and electrochemical hydrogen evolution, Appl. Catal., B., 228 (2018) 64-74.

SC RI PT

[19] R. Abe, Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation, J. Photoch. Photobio. C, 11 (2010) 179-209.

[20] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev., 43 (2014) 7520-7535.

[21] R. Raja, P. Sudhagar, A. Devadoss, C. Terashima, L.K. Shrestha, K. Nakata, R. Jayavel,

U

K. Ariga, A. Fujishima, Pt-free solar driven photoelectrochemical hydrogen fuel

A

Commun (Camb), 51 (2015) 522-525.

N

generation using 1T MoS2 co-catalyst assembled CdS QDs/TiO2 photoelectrode, Chem

M

[22] Y. Pi, Z. Li, D. Xu, J. Liu, Y. Li, F. Zhang, G. Zhang, W. Peng, X. Fan, 1T-Phase MoS2 Nanosheets on TiO2 nanorod arrays: 3D photoanode with extraordinary catalytic

D

performance, ACS Sustain. Chem. Eng., 5 (2017) 5175-5182.

TE

[23] X. Zhu, X. Liang, X. Fan, X. Su, Fabrication of flower-like MoS2/TiO2 hybrid as an anode

EP

material for lithium ion batteries, RSC Adv., 7 (2017) 38119-38124. [24] J. Zhang, L. Zhang, W. Yu, F. Jiang, E. Zhang, H. Wang, Z. Kong, J. Xi, Z. Ji, Novel dual

CC

heterojunction between MoS2 and anatase TiO2 with coexposed {101} and {001} facets, J. Am. Ceram. Soc., 100 (2017) 5274-5285.

A

[25] B. Chen, Y. Meng, J. Sha, C. Zhong, W. Hu, N. Zhao, Preparation of MoS 2/TiO2 based nanocomposites for photocatalysis and rechargeable batteries: progress, challenges, and perspective, Nanoscale, 10 (2017) 34-68.

20

[26] H. He, J. Lin, W. Fu, X. Wang, H. Wang, Q. Zeng, Q. Gu, Y. Li, C. Yan, B.K. Tay, C. Xue, X. Hu, S.T. Pantelides, W. Zhou, Z. Liu, MoS2/TiO2 Edge-on heterostructure for efficient photocatalytic hydrogen evolution, Adv. Energy Mater., 6 (2016) 1600464.

SC RI PT

[27] L. Guo, Z. Yang, K. Marcus, Z. Li, B. Luo, L. Zhou, X. Wang, Y. Du, Y. Yang, MoS2/TiO2 heterostructures as nonmetal plasmonic photocatalysts for highly efficient hydrogen evolution, Energy Environ. Sci., 11 (2018) 106-114.

[28] W.K. Wang, J.J. Chen, X. Zhang, Y.X. Huang, W.W. Li, H.Q. Yu, Self-induced synthesis of phase-junction TiO2 with a tailored rutile to anatase ratio below phase transition

U

temperature, Sci. Rep., 6 (2016) 20491.

N

[29] A. Li, Z. Wang, H. Yin, S. Wang, P. Yan, B. Huang, X. Wang, R. Li, X. Zong, H. Han,

A

C. Li, Understanding the anatase–rutile phase junction in charge separation and transfer

M

in a TiO2 electrode for photoelectrochemical water splitting, Chem. Sci., 7 (2016) 60766082.

D

[30] Y. Zhao, N. Hoivik, K. Wang, Recent advance on engineering titanium dioxide nanotubes

EP

728-744.

TE

for photochemical and photoelectrochemical water splitting, Nano Energy, 30 (2016)

[31] F.F. Abdi, L. Han, A.H. Smets, M. Zeman, B. Dam, R. van de Krol, Efficient solar water

CC

splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode, Nat. Commun., 4 (2013) 2195.

A

[32] N. Wang, M. Liu, H. Tan, J. Liang, Q. Zhang, C. Wei, Y. Zhao, E.H. Sargent, X. Zhang, Compound homojunction: heterojunction reduces bulk and interface recombination in ZnO photoanodes for water splitting, Small, 13 (2017).

21

[33] Z. Zhang, Y. Huang, K. Liu, L. Guo, Q. Yuan, B. Dong, Multichannel-improved chargecarrier dynamics in well-designed hetero-nanostructural plasmonic photocatalysts toward highly efficient solar-to-fuels conversion, Adv. Mater., 27 (2015) 5906-5914.

SC RI PT

[34] Y.H. Tseng, C.S. Kuo, C.H. Huang, Y.Y. Li, P.W. Chou, C.L. Cheng, M.S. Wong, Visible-light-responsive nano-TiO(2) with mixed crystal lattice and its photocatalytic activity, Nanotechnology, 17 (2006) 2490-2497.

[35] M.A. Mohamed, W.N. Wan Salleh, J. Jaafar, N. Yusof, Preparation and photocatalytic activity of mixed phase anatase/rutile TiO2 nanoparticles for phenol degradation, Jurnal

U

Teknologi, 70 (2014).

N

[36] S. Yeniyol, I. Mutlu, Z. He, B. Yuksel, R.J. Boylan, M. Urgen, Z.C. Karabuda, C.

A

Basegmez, J.L. Ricci, Photocatalytical antibacterial activity of mixed-phase TiO2

M

Nanocomposite thin films against Aggregatibacter actinomycetemcomitans, BioMed research international, 2015 (2015) 705871.

D

[37] R. Jiang, B. Li, C. Fang, J. Wang, Metal/Semiconductor hybrid nanostructures for

TE

plasmon-enhanced applications, Adv. Mater., 26 (2014) 5274-5309.

EP

[38] Y. Pihosh, I. Turkevych, K. Mawatari, N. Fukuda, R. Ohta, M. Tosa, K. Shimamura, E.G. Villora, T. Kitamori, Ubiquitous element approach to plasmonic enhanced photocatalytic

CC

water splitting: the case of Ti@TiO2 core-shell nanostructure, Nanotechnology, 25 (2014) 315402.

A

[39] G. Liu, K. Du, J. Xu, G. Chen, M. Gu, C. Yang, K. Wang, H. Jakobsen, Plasmondominated photoelectrodes for solar water splitting, J. Mater. Chem. A, 5 (2017) 42334253.

22

[40] S. Tan, A. Argondizzo, J. Ren, L. Liu, J. Zhao, H. Petek, Plasmonic coupling at a metal/semiconductor interface, Nat. Photonics, 11 (2017) 806-812. [41] M.W. Knight, N.S. King, L. Liu, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum for

SC RI PT

plasmonics, ACS nano, 8 (2014) 834-840. [42] N.T. Thomas, K. Nobe, Kinetics of the hydrogen evolution reaction on titanium, J. Electrochem. Soc., 117 (1970) 622.

[43] D. Kiriya, P. Lobaccaro, H.Y. Nyein, P. Taheri, M. Hettick, H. Shiraki, C.M. Sutter-Fella, P. Zhao, W. Gao, R. Maboudian, J.W. Ager, A. Javey, General thermal texturization

U

process of MoS2 for efficient electrocatalytic hydrogen evolution reaction, Nano Lett., 16

N

(2016) 4047-4053.

A

[44] X. Zou, Y. Zhang, Noble metal-free hydrogen evolution catalysts for water splitting,

M

Chem. Soc. Rev., 44 (2015) 5148-5180.

[45] J. Kibsgaard, Z. Chen, B.N. Reinecke, T.F. Jaramillo, Engineering the surface structure

TE

(2012) 963-969.

D

of MoS2 to preferentially expose active edge sites for electrocatalysis, Nat. Mater., 11

EP

[46] X. Hai, W. Zhou, K. Chang, H. Pang, H. Liu, L. Shi, F. Ichihara, J. Ye, Engineering the crystallinity of MoS2 monolayers for highly efficient solar hydrogen production, J. Mater.

CC

Chem. A, 5 (2017) 8591-8598.

A

[47] K. Chang, H. Pang, X. Hai, G. Zhao, H. Zhang, L. Shi, F. Ichihara, J. Ye, Ultra-small freestanding amorphous molybdenum sulfide colloidal nanodots for highly efficient photocatalytic hydrogen evolution reaction, Appl. Catal., B., 232 (2018) 446-453.

[48] C. Tan, Z. Luo, A. Chaturvedi, Y. Cai, Y. Du, Y. Gong, Y. Huang, Z. Lai, X. Zhang, L. Zheng, X. Qi, M.H. Goh, J. Wang, S. Han, X.J. Wu, L. Gu, C. Kloc, H. Zhang, Preparation

23

of

high-percentage

1T-phase

transition

metal

dichalcogenide

nanodots

for

electrochemical hydrogen evolution, Adv. Mater., 30 (2018). [49] M.A.R. Anjum, H.Y. Jeong, M.H. Lee, H.S. Shin, J.S. Lee, Efficient hydrogen evolution

SC RI PT

reaction catalysis in alkaline media by all-in-one MoS2 with multifunctional active sites, Adv. Mater., 30 (2018) e1707105.

[50] E. Parzinger, B. Miller, B. Blaschke, J.A. Garrido, J.W. Ager, A. Holleitner, U. Wurstbauer, Photocatalytic stability of single- and few-layer MoS(2), ACS nano, 9 (2015) 11302-11309.

U

[51] T. Lopes, L. Andrade, H.A. Ribeiro, A. Mendes, Characterization of photoelectrochemical

N

cells for water splitting by electrochemical impedance spectroscopy, Int. J. Hydrogen

A

Energy, 35 (2010) 11601-11608.

M

[52] M.I.D.-G.a.R. Gómez, Investigating water splitting with CaFe2O4 photocathodes by electrochemical impedance spectroscopy, ACS Appl. Mater. Interfaces, 8 (2016) 11.

D

[53] Y.-J. Yuan, Z.-J. Ye, H.-W. Lu, B. Hu, Y.-H. Li, D.-Q. Chen, J.-S. Zhong, Z.-T. Yu, Z.-

TE

G. Zou, Constructing anatase TiO2 nanosheets with exposed (001) facets/layered MoS2

EP

two-dimensional nanojunctions for enhanced solar hydrogen generation, ACS Catal., 6 (2015) 532-541.

CC

[54] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley,

A

C.R. Catlow, M.J. Powell, R.G. Palgrave, I.P. Parkin, G.W. Watson, T.W. Keal, P. Sherwood, A. Walsh, A.A. Sokol, Band alignment of rutile and anatase TiO(2), Nat. Mater., 12 (2013) 798-801.

[55] N. Baram, Y. Ein-Eli, Electrochemical impedance spectroscopy of porous TiO2 for photocatalytic applications, J. Phys. Chem. C, 114 (2010) 9781-9790.

24

[56] L. Cao, R. Wang, D. Wang, X. Li, H. Jia, MoS2 -hybridized TiO2 nanosheets with exposed {001} facets to enhance the visible-light photocatalytic activity, Mater. Lett., 160 (2015) 286-290.

SC RI PT

[57] J. Tao, J. Chai, L. Guan, J. Pan, S. Wang, Effect of interfacial coupling on photocatalytic performance of large scale MoS2/TiO2 hetero-thin films, Appl. Phys. Lett., 106 (2015) 081602.

[58] D. Wang, Y. Xu, F. Sun, Q. Zhang, P. Wang, X. Wang, Enhanced photocatalytic activity of TiO2 under sunlight by MoS2 nanodots modification, Appl. Surf. Sci., 377 (2016) 221-

U

227.

N

[59] X. Liu, Z. Xing, Y. Zhang, Z. Li, X. Wu, S. Tan, X. Yu, Q. Zhu, W. Zhou, Fabrication of

A

3D flower-like black N-TiO2-x @MoS2 for unprecedented-high visible-light-driven

M

photocatalytic performance, Appl. Catal. B: Environ., 201 (2017) 119-127. [60] Z. Jiang, W. Wan, H. Li, S. Yuan, H. Zhao, P.K. Wong, A hierarchical Z-scheme alpha-

D

FeO3/g-C3N4 hybrid for enhanced photocatalytic CO2 reduction, Adv. Mater., (2018).

TE

[61] X. Yang, H. Huang, B. Jin, J. Luo, X. Zhou, Facile synthesis of MoS2/B-TiO2 nanosheets

EP

with exposed {001} facets and enhanced visible-light-driven photocatalytic H2 production activity, RSC Adv., 6 (2016) 107075-107080.

CC

[62] Y. Yang, Y. Ling, G. Wang, T. Liu, F. Wang, T. Zhai, Y. Tong, Y. Li, Photohole Induced

A

Corrosion of Titanium Dioxide: Mechanism and Solutions, Nano Lett., 15 (2015) 70517057.

[63] D. McAteer, Z. Gholamvand, N. McEvoy, A. Harvey, E. O'Malley, G.S. Duesberg, J.N. Coleman, Thickness dependence and percolation scaling of hydrogen production rate in

25

MoS2 nanosheet and nanosheet-carbon nanotube composite catalytic electrodes, ACS Nano, 10 (2016) 672-683. [64] Y. Zhang, H. Li, H. Wang, H. Xie, R. Liu, S.L. Zhang, Z.J. Qiu, Thickness considerations

SC RI PT

of two-dimensional layered semiconductors for transistor applications, Sci. Rep., 6 (2016) 29615.

[65] A. Sohn, H. Moon, J. Kim, M. Seo, K.-A. Min, S.W. Lee, S. Yoon, S. Hong, D.-W. Kim, Band alignment at Au/MoS2 contacts: thickness dependence of exfoliated flakes, J. Phys.

A

CC

EP

TE

D

M

A

N

U

Chem. C, 121 (2017) 22517-22522.

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D

M

A

N

U

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Figures (color):

Figure 1. a). XRD patterns of the as-prepared samples. b) Orientation map of mixed-phases TiO2

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marked with assorted colors using EBSD microstructural crystallographic technology. More parameters refer to Figure S3 (Table S1, S2 and S3). (c) Ultraviolet-visible (UV-Vis) absorption

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spectrums of all samples TiO2/Ti after 2 hours annealing process at respective 400, 500 and 600 ℃.

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(d) UV-Vis absorption spectra of a group of TiO2/Ti samples after 2 hours of annealing under varied temperature with distinct visible light absorption bandwidth.

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Figure 2. (a-c) Room-temperature J-V curves of the TiO2/Ti measured in 0.5 M H2SO4 under

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respective dark and light with AM 1.5G illumination (300 W/m2). (d) Tafel plots corresponding to

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the HER polarization curves and related data for the samples.

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Figure 3. (a). Schematic fabrication processes of all samples. Five kinds of samples involved in

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the experiments: Ti foil (sample A), TiO2/Ti (sample B), pristine MoS2/TiO2/Ti (sample C), and annealed MoS2/TiO2/Ti in nitrogen atmosphere at temperatures of 500 ℃ (1h/2h/3h) (sample

A

D/E/F). The corresponding SEM images (b) and X-Ray diffraction patterns (c) of the as-received samples.

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Figure 4. (a) Polarization curves for the composites of MoS2/Ti, measured in 0.5 M H2SO4 at a

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scanning rate of 20 mV/S under the light. (b) The corresponding Tafel plots and related data for

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the MoS2/Ti (500 2h). The long-term stability test for (c) the MoS2/Ti (500 ℃ 2h), (d) MoS2/TiO2 under the dark and (e) MoS2/Ti under the light, respectively, during the HER at -0.3 V vs RHE for 15 hours. (f) The HER polarization curves of the MoS2/Ti (500 2h) measured before and after the 10 hours stability test at 20 mV/S. The figure of 500 in the figures all refers to the temperature of 500 ℃.

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Figure 5. Energy diagram of charge-carrier generation, separation and transportation processes

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for the hybrid-junction MoS2/TiO2 on the Ti substrate. (1) Plasmons assisted charge-carrier

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injection; (2) band-gap enabled excitation of charge-carrier; (3) Z-scheme mechanism of charge-

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carrier transportation.

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